The present invention relates to a method of cooling an optical fiber during drawing through contact with at least one cooling fluid in at least one cooling area.
There are various categories of optical fiber: optical fibers based on oxide glass, optical fibers based on fluoride glass, and plastics material optical fibers based on polymer materials. Optical fiber based on oxide glass, usually silica glass, is manufactured by drawing a heated preform, which is a large cylinder of silica glass, optionally at least partly doped, whose diameter generally lies in the range 20 mm to 200 mm and whose length generally lies in the range 300 mm to 2000 mm. FIG. 1 is a diagrammatic view of a drawing tower 1. A preform 2 is melted in a drawing furnace 3 which heats the preform to a temperature of approximately 2000xc2x0 C. A fiber 7 obtained in this way is cooled initially by the surrounding air, then in at least one cooling device 4, and finally by the surrounding air again, before it is fed into a coating device 5. The position of the cooling device 4 in the drawing tower 1 is generally optimized to obtain the correct fiber temperature for resin coating. The coating device 5 forms the coating of the fiber 7 from at least one coating resin which is usually cured by ultraviolet light. The device 5 generally includes at least one injection device (5a, 5c) followed by at least one curing device (5b, 5d). In the situation shown in FIG. 1, the device 5 includes a primary resin injection device 5a followed by a device 5b for curing said resin by ultraviolet light, and then a secondary resin injection device 5c followed by a device 5d or curing said resin by ultraviolet light. Finally, a coated optical fiber 8 is pulled by a capstan 6 and then wound onto a take-up spool 9.
The devices under the drawing furnace 3, which are on a common downward vertical axis Z, are generally identified by their position relative to the bottom of the drawing furnace 3, as indicated by the dimension z. All the components of the device shown in FIG. 1 are well-known to the person skilled in the art. Others, which are not shown, are also well-known to the person skilled in the art. Thus, for example, means for measuring the diameter of the bare and/or coated fiber, means for measuring the eccentricity of the fiber within its primary and/or secondary coating, and means for measuring the temperature of the fiber at a given distance along the axis are part of the prior art.
Cooling must reduce the temperature of the fiber leaving the drawing furnace to a temperature compatible with application of the coating resin, i.e. a temperature of the order of 50xc2x0 C. The temperature of the fiber leaving the drawing furnace is high, generally of the order of 1000xc2x0 C. to 2000xc2x0 C. for a silica-based fiber, depending on the drawing furnace and the drawing speed used. Cooling the fiber between leaving the drawing furnace and entering the coating device is one of the major problems to be solved in drawing fibers, especially if it is required to increase the drawing speed. It is well-known that the attenuation of the fiber depends on the cooling conditions, and moreover, if the temperature of the fiber on entering the coating device is too high, this can lead to problems both with the eccentricity of the fiber in its coating and with the quality of said coating. The speed at which silica-based fibers are drawn industrially, which was 300 meters per minute (m/min) a few years ago, has increased more and more, and is now of the order of 1500 m/min or more. This tendency is still apparent, associated with increasing productivity, which is one of the major objectives of the optical fiber industry.
The principle of the process for fabricating optical fibers based on fluoride glass is the same, but the preform is generally smaller, generally having a diameter of 15 mm to 20 mm and a maximum length of a few centimeters to a few tens of centimeters, for example 10 cm, and the temperature on leaving the drawing furnace generally lies in the range 300xc2x0 C. to 450xc2x0 C. The same technical problem can arise in this case. Similarly, the same technical problem can arise in the fabrication of optical fibers based on polymer materials, in which the preform generally has a diameter of a few tens of millimeters, for example 80 mm, and a maximum length of a few tens of centimeters, for example 50 cm, and the temperature on leaving the drawing furnace generally lies in the range 200xc2x0 C. to 250xc2x0 C. The remainder of the description refers to optical fibers based on silica, but identical reasoning applies to other types of optical fiber, including optical fibers based on oxide glasses other than silica.
Various devices have been used to cool silica-based fiber. One solution would be to increase the area of heat exchange between the fiber to be cooled and the surrounding air, in particular by increasing the distance between the drawing furnace and the coating device. However, this would entail increasing the height of the drawing towers currently used, which would be much too costly, especially in terms of the investment required.
Another solution is to improve the efficiency of cooling over the existing distance between the drawing furnace and the coating device. In addition to simple cooling by the surrounding air, which proves to be highly inadequate for the drawing towers currently used, the common principle of various devices used in the industry (as illustrated by European Patent Application EP-A1-0 079 186, for example) consists in injecting a gas radially towards the surface of the fiber at a given distance from the outlet of the drawing furnace and causing said gas to flow upwards or downwards over a particular length of the fiber, inside a heat exchange tube. As is well-known to the person skilled in the art, heat is transferred because of the thermal conductivity of said gas, which gas is generally air, carbon dioxide, nitrogen, argon, or helium, and is preferably helium. The periphery of the tube is preferably cooled by a cooling fluid, which is generally water. By way of example, U.S. Pat. No. 4,761,168 describes an improvement to such systems in which the gas is caused to flow along the fiber in a heat exchange tube of particular shape, which ensures regular renewal of the boundary layer of gas flowing along the fiber. The improvement is aimed at improving the efficiency of heat exchange.
One of the main problems encountered in subsequent use of optical fiber cooled in the above way is that the cooling imposed on the fiber during its fabrication, on leaving the drawing furnace and before passing through the coating furnace, significantly increases the level of Rayleigh back scattering associated with the fiber and therefore increases the major part of the attenuation of the optical fiber ready for use. It is known in the art that the attenuation of optical fiber at the wavelengths used, whether close to 1310 nm or to 1550 nm, must be as low as possible for optimum transmission of optical signals in said fiber.
That is why several solutions have been proposed to the problem of defining cooling profiles which are obtained by particular methods and/or devices and which minimize Rayleigh back scattering in the fiber. At least partial use of slow cooling profiles is generally proposed, meaning profiles that are slower than those obtained for cooling by the surrounding air. Patent Application DE-A1-3 713 029, for example, teaches slow cooling on leaving the drawing furnace.
Such methods are not satisfactory, however, in that they do not achieve sufficient reduction of the attenuation compared to the theoretical minimum attenuation.
An object of the present invention is to alleviate the above drawbacks of prior art cooling systems by improving the cooling of an optical fiber during drawing. One particular object of the invention is to reduce Rayleigh back scattering significantly, compared to prior art cooling systems, and therefore to reduce the attenuation of the fiber fabricated by drawing using the cooling method of the invention.
To this end, the invention provides a method of cooling an optical fiber during drawing, through contact with at least one cooling fluid in at least one cooling area, the method comprising effecting fast cooling, i.e. cooling that is faster than cooling in the surrounding air, from an initial temperature of the fiber to an intermediate temperature of said fiber in a fast cooling area, followed by slow cooling, i.e. cooling slower than cooling in the surrounding air, from an intermediate temperature of the fiber to a final temperature of said fiber, in a slow cooling area, the temperature of the fiber in an intermediate area between the two cooling areas lying in the range 1200xc2x0 C. to 1700xc2x0 C. in the case of fibers based on silica glass, in the range 200xc2x0 C. to 400xc2x0 C. in the case of fibers based on fluoride glass, or in the range 150xc2x0 C. to 250xc2x0 C. in the case of fibers based on polymer materials.
The temperature of the fiber in the intermediate area is preferably substantially equal to a xe2x80x9ccriticalxe2x80x9d temperature such that the attenuation of the fiber obtained by the drawing method is a minimum.
In other words, the above temperature, referred to as the critical temperature, is related to the glass transition temperature associated with Rayleigh back scattering in the core of the fiber. The glass transition temperature associated with Rayleigh back scattering is a thermodynamic parameter characteristic of the disorder of the glass, and more precisely of variations in density which contribute to Rayleigh back scattering. The state of disorder achieved during solidification of the glass of the core of the fiber is that found in the finished fiber after drawing and the aim is to reduce the variations in density. The method of the invention advantageously provides better control over cooling of the optical fiber, which minimizes variations in the density of the glass.
One advantage of the method of the invention is that it complies with economic constraints, which limit the height of the tower available for cooling and require high drawing speeds. The presence of a fast cooling area eliminates the problem of the height of the drawing tower and/or the drawing speed.
Another advantage of the method of the invention is that a fast cooling area between the drawing furnace and a slow cooling area significantly improves the attenuation of the fiber obtained by the method of the invention, other conditions being equal. It is important to apply the fast cooling over a range of temperatures higher than said critical temperature, because if quenching is effected at the critical temperature, or at a lower temperature, the effect on Rayleigh back scattering can be disastrous and the attenuation of the fiber finally obtained can be greatly increased.
The length of fiber in the intermediate area is preferably as short as possible, given the technological constraints on the drawing tower.
The fast cooling of the fast cooling area is at least as fast, and preferably faster, than cooling in the surrounding air, at least locally in the vicinity of the critical temperature. In other words, the instantaneous slope dT/dt of fast cooling, where T is the temperature of the fiber and t is time, has, at least locally in the vicinity of the critical temperature, a higher absolute value for such cooling than said instantaneous slope for cooling the fiber in the surrounding air. Said instantaneous slope has a higher absolute value for such cooling than said instantaneous slope for cooling the fiber in the surrounding air, preferably on average in the fast cooling area, more preferably in the major part of the fast cooling area, and even more preferably in virtually all of the fast cooling area.
The slow cooling of the slow cooling area is at least as slow, and preferably slower, than cooling in the surrounding air, at least locally, in the vicinity of the critical temperature. As a general rule, the instantaneous slope dT/dt of slow cooling, where T is the temperature of the fiber and t is time, has, at least locally in the vicinity of the critical temperature, a lower absolute value for such cooling than said instantaneous slope for cooling the fiber in the surrounding air. Said instantaneous slope generally has a lower absolute value for such cooling than said instantaneous slope for cooling the fiber in the surrounding air, preferably on average in the slow cooling area, more preferably in the major part of the slow cooling area, and even more preferably in virtually all of the slow cooling area.
In one embodiment, at least locally in the vicinity of the critical temperature, the ratio of the instantaneous slopes dT/dt for fast cooling in the fast cooling area and for cooling in the surrounding air, where T is the temperature of the fiber and t is time, is generally greater than 1, preferably greater than 1.1, and more preferably in the range 1.2 to 10.
The initial temperature of the fiber at the entry of the fast cooling area is advantageously approximately equal to the critical temperature, generally plus 250xc2x0 C. to 350xc2x0 C., typically plus approximately 300xc2x0 C.
In one embodiment, at least locally in the vicinity of the critical temperature, the ratio of the instantaneous slopes dT/dt for slow cooling in the slow cooling area and for cooling in the surrounding air, where T is the temperature of the fiber and t is time, is generally less than 1, preferably less than 0.9, and more preferably lies in the range 0.05 to 0.8 in the slow cooling area. However, in the context of the invention, the slow cooling area can also be a heating area, i.e. in at least part of said slow cooling area the ratio of the instantaneous slopes dT/dt for slow cooling and for cooling in the surrounding air, where T is the temperature of the fiber and t is time, can be negative.
The final temperature of the fiber on leaving the slow cooling area is advantageously approximately equal to the critical temperature, generally minus 50xc2x0 C. to 950xc2x0 C., typically minus approximately 500xc2x0 C.
In the case of an ideal structural model of the glass, the slope dT/dt in the fast cooling area must ideally be as steep as possible, i.e. quasi-infinite; conversely, the slope dT/dt in the slow cooling area must ideally be as level as possible, i.e. quasi-zero. The limitations of the material, the constraints of the device and the non-ideal nature of the actual vitreous structure lead to said slopes having intermediate values, corresponding to the values indicated hereinabove. The exact temperature profiles within the fast and slow cooling areas could be adapted more closely to obtain the maximum reduction of attenuation, for example by sub-dividing said areas into sub-areas with locally tailored slopes. In the example described hereinafter, the device is restricted to two homogeneous areas.
The critical temperature depends primarily on the composition of the fiber. The applicant has determined that, for a standard silica-based stepped index optical fiber conforming to CCITT Standard G.652, said critical temperature generally lies in the range 1350xc2x0 C. to 1550xc2x0 C. and preferably in the range 1450xc2x0 C. to 1550xc2x0 C. It is 1500xc2x120xc2x0 C., for example.
The present invention, although described with reference to silica glass fibers, applies equally to the other categories of fiber described previously, the temperature ranges being adapted accordingly by the person skilled in the art.
Cooling, whether fast or slow, is effected by any device known to the person skilled in the art. The cooling fluid is generally chosen from the group comprising air, carbon dioxide, argon, nitrogen and helium. Said fluid is preferably helium.